A person having familiarity with black liquor recovery boilers would know that they are necessary parts of the pulp production process and are used in pulp mills for liquor disposal, steam generation and recovery of chemical products through a combustion process. The production of a pulp mill is integrally linked to its recovery boiler's liquor firing and steam generating capacity.
In recent times, pulp mills have been gradually increasing their level of production. This has been occurring primarily as a result of better process management and more efficient use of existing machinery.
These improvements have come about in the form of upgrades in existing mills. When these mills were originally built, for example, they were built with recovery boilers that were adequate to meet mill capacity at that time. However, mill upgrades are now pushing recovery boiler use requirements beyond their original design capacity. As a result, recovery boilers are now limiting and creating a major restriction in pulp mill output.
The solution to this problem is for mills to either build one or more additional recovery boilers, or modify their existing boilers. The former is certainly technically feasible although it is expensive and therefore represents an economic detriment. The latter has not always been technically or economically feasible. The economics of pulp making is dictating a need to develop cost-effective ways of upgrading the capacity of existing boilers and this is the underlying reason leading to the development of the present invention.
The main objective in recovery boilers is to dispose of a process waste material (black liquor) by burning the organic residue, thereby generating steam, and converting the inorganic chemicals to a reusable form. This is to be done while at the same time minimizing the carry-over of particulate matter and release of environmentally-objectionable gases through the boiler's stack. There have been many attempts in the past to improve boiler efficiency by implementing complex control systems that affect airflow rate into the combustion chamber. Notable examples of this are shown in U.S. Pat. No. 4,362,269 issued to Rastogi on Dec. 7, 1982, and U.S. Pat. No. 4,359,950 issued to Leffler on Nov. 23, 1982.
Both patents provide a good description of recovery boiler operation and recognize that boiler efficiency is affected by the control of air into the combustion chamber. Nevertheless, both are primarily concerned with the chemical efficiency of the combustion process which, of course, is also a concern in the present invention. However, neither patent is concerned with increasing boiler capacity in the same way as the present invention or, in other words, neither provides combustion air in the same way as the present invention.
A significant difference between the present invention and the known prior art is that the prior art does not force a penetrating airflow into the furnace in the central part of the boiler above the burning bed. This is the crux of the invention disclosed herein, and it creates turbulent mixing with the furnace gases above the bed, which significantly improves boiler combustion efficiency and capacity.